Why does sweeping the ice improve gliding?

What's going on under the broom?

Watching curling, haven't you ever wondered why they are sweeping the ice when it seems free of obstructions?

Curling was originally an outdoor sport, and sweeping began as a way of clearing the ice. In indoor play, though, it is no longer necessary to sweep the ice clean. Why, then, do they sweep?

The answer is to reduce friction between the stone and the ice. Since ice is extremely slippery, you might not think there would be much friction, but there is some.

Sweeping the surface of the ice has the obvious effect of smoothing out little bumps, reducing friction and making the stone glide over it more easily. But there's another explanation, too: the sweeping action of the brooms generates friction and thus heat, making the ice melt slightly. That slight layer of water may help the stone glide faster, though that explanation has yet to be proven.

Pebbled ice influences the outcome of a game

Which do you think it would be easier to glide on, (1) ice that is frozen very hard, or (2) ice that is slightly thawed on top?

The answer is (2). Generally speaking, little friction occurs between ice or snow and your foot, for instance; that's why they're slipper. But ice that's thawing slightly has a film of water on its surface that acts as a lubricant, making it even easier to slip--or glide--on it.

To counter that effect, the sheet of ice used for curling is treated in a special way. Before a curling event begins, water is sprinkled on the rink from what looks like a watering can. That gives the ice a pebbled surface that slows the stones down. The outcome of a game then depends in part on how skillful the players are at getting rid of those pebbles.